The present invention relates to the dewatering of sludges, slurries, or the like, that contain macro-molecules, colloidal particles, and/or suspended particles in a carrier liquid, usually water, and, more particularly, to dewatering of such materials using electrokinetic techniques including both electrophoretic and electro-osmotic techniques.
In addition, this invention relates to the application of the electrokinetic techniques disclosed to such electrokinetic processes as electrodialysis, including electrolysis, desalinization, and metathesis; and electrowinning; as well as to electrophoretic processes other than the dewatering of sludges and slurries.
Many industries, especially the paper making and related fiber production industries, generate large quantities of sludge or slurry-like effluents that contain various concentrations of materials generically described as "solids" that can include, for example, dispersed and semi-dispersed macro-molecules, colloidal particles, and suspended particulate matter. As part of the treatment of the effluents, the liquid carrier or water is separated from the solids with the so-separated water being recycled to the industrial process or returned to the environment and the solids disposed of through landfill operations.
In the past, the dewatering of effluents to isolate solids has been accomplished by thermal evaporation, mechanical filtration, chemical processing, settling, flotation, and a variety of other methods. In some special cases, electrical dewatering of effluents and sludges has been accomplished by establishing an electric field in the sludge between electrodes immersed within the sludge. As a consequence of the applied electric field, particles within the sludge that carry a net positive or negative charge, including macro-molecules, colloidal particles, and suspended particles, electrophoretically migrate to their respective, oppositely charged, counter electrode. In addition to the migration of the particulate matter, water molecules which are bound to or otherwise associated with some of the charged particles will co-migrate with those particles. As a consequence of this migration of charged particles and associated water molecules, the solid particles consolidate and densify to effect partial separation from the liquid carrier. Filtration devices can be provided at or near one of the electrodes to permit removal of the water by electro-osmotic techniques. With continued application of the electric field and water removal, the solids concentration of sludge increases to the point where the solids densify and consolidate into a load-bearing material to complete the dewatering.
The principal economic cost of electrokinetic dewatering has been the cost of electricity for generating and sustaining the applied electric field. In the past, direct current has been applied to the electrodes to establish the electric field. Direct current has the advantage of establishing a uniform field but has the associated disadvantage of not being readily available from power mains and, accordingly, must be generated on site, usually through motor-generator sets. Experience indicates that the dewatering rate, that is, the rate of water removal over time, or conversely, the rate of solids consolidation, is roughly proportional to the power consumed in kilowatt hours (KWH). The economic costs associated with dewatering an acre of sludge as part of a landfill operation are presently between $8500-$9500 (1980 dollars). Accordingly, there is an economic need to effect dewatering by electrokinetic techniques at substantially reduced electrical energy costs.
In addition, there is a continuing need in the art of electrokinetic dewatering to dewater sludges continuously in a rapid, cost-effective manner, in situ, to ease the burden of sludge handling and disposal of material reclamation. In addition, it is desirable to be able to adapt existing conventional continuous sludge conveyances readily to utilize the techniques of this invention. It is toward the solution of these aspects of the problems that this application is directed, as well as to the application of the techniques described in the parent case to a continuous dewatering system.
In addition to the application of the teachings of this invention to electrophoretic dewatering of liquid of the type which includes emulsions, slurries, and solutions, the advantages of the invention are expected to be obtained for the electrophoretic treatment of such compositions as polyvinyl acetate, polyvinyl chloride, and other compounds manufactured, purified or treated using electrophoresis. Thus, it is expected that the application of a time-varying, intermittent, unidirectional current, having the wave form here described, to cathode and anode electrodes minimizes the consumption of power in electrophoretic processes experiencing the phenomenon of migration of suspended or colloidal particles in a liquid due to an emf or potential applied to the electrodes.
In addition to its application to electrophoretic processes, it is expected that the invention may also be used with processes using electrodialysis and electrowinning. Examples of types of electrodialysis processes to which the invention can be applied are processes using electrolysis, whether using a membrane process or a bipolar membrane, such as in the manufacture of chlorine or chlorination processes; desalinization processes, especially those using a membrane; and metathesis processes. An example of metathesis is the typical reaction: CAOH.sub.2 +2NACL=CACL.sub.2 +2NAOH.
Electrolysis or electrolytic conduction, for purposes of the application of the teachings of this invention, can be considered to be the conduction of electricity accompanied by the actual transfer of matter, i.e. migration of ions, which is shown by the occurrence of chemical changes at the electrodes.
Electrowinning is the recovery of metals from ores by electrochemical processes and is also known as electro-extraction. In particular, such processes involve the recovery of a metal from a solution of its salts by passing an electrical current through the solution. Electrowinning is thus a common process for extracting metal from its ore. Electrowinning processes may also involve electroplating which is the production of a thin coating of one metal on another by electrodeposition by making the respective metals the anode and cathode in an electrolytic cell containing a solution of a salt of the metal to be deposited.
Thus, it is believed that to minimize the power consumption the invention can be applied to a wide variety of processes which conventionally have used direct current to perform the process to achieve an electrokinetic treatment of matter.